1. Field of the Invention
The present invention relates to Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication, and more specifically, to an integer carrier frequency offset (ICFO) estimation for OFDMA wireless communication, especially in the Worldwide Interoperability for Microwave Access (WiMAX) system.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a promising technology that has been widely used in wireless communication systems, and converts a frequency selective channel to a collection of frequency-flat subchannels achieved by splitting an input high-rate data stream into a number of low-rate substreams. In order to enhance the efficiency of the frequency allocation, the Orthogonal Frequency Division Multiple Access (OFDMA) technique is proposed to allow multiple users to access a same channel, by dividing available subcarriers into mutually exclusive clusters assigned to distinct users for simultaneous transmission. The orthogonality of the subcarriers guarantees intrinsic protection against multiple access interference, while the adoption of a dynamic subcarrier assignment strategy provides the system with high flexibility in resource management. OFDMA has become apart of IEEE 802.16 standards for wireless metropolitan area networks as a promising candidate for next generation broadband wireless networks.
OFDMA is extremely sensitive to timing error and carrier frequency offset (CFO) between an incoming signal and local references used for signal demodulation. The time error affects the FFT window and produces inter-symbol interference (ISI). Inaccurate compensation of the carrier frequency offset destroys orthogonality among subcarriers and produces inter-carrier interference (ICI). Usually the carrier frequency offset is divided into two parts: fractional carrier frequency offset (FCFO) and integer carrier frequency offset (ICFO). FCFO destroys the orthogonality among the subcarriers, while the ICFO leads to a frequency shift with several subcarriers frequency spacing.
In an OFDMA system, ICFO is a coarse frequency offset and shows the number of subcarriers shift in an original spectrum in a frequency domain. Without ICFO compensation, there may be 50 percent of bit error ratio in a post-processing decoder of an OFDMA receiver. Therefore, ICFO estimation is an important part of the synchronization process in a downlink receiver.
There have been many algorithms for performing carrier frequency offset estimation. However, most of them are proposed for the FCFO estimation. FCFO is often estimated by time-domain correlation while ICFO is estimated in a frequency domain.
A well-known method is disclosed in T. M. Schmidl and D. C. Cox's “Robust Frequency and Timing Synchronization for OFDM” (IEEE Trans. Commun., Vol. 45, No. 12, pp. 1613-1621, December 1997). In this method, correlations of the subcarriers between two consecutive OFDM symbols for the coarse estimation are performed. In addition, correlations have to be conducted for all possible integer frequency offsets in predetermined estimation ranges. As a result, such a method leads to computational complexity and is not available in actual applications.
An improved method is presented in Bo-Seok Seo and Su-Chang Kim's “Fast Coarse Frequency Offset Estimation for OFDM Systems By Using Differentially Modulated Subcarriers” (IEEE Transactions on Consumer Electronics, Vol. 48, No. 4, pp. 1075-1081, November 2002). It uses the phase difference of the adjacent subcarriers in one OFDM symbol to do correlation, and the integer frequency offset is obtained from the correlation phase difference between two consecutive symbols. But in an OFDMA WiMAX system, the preamble has only one symbol and shows no property of fixed differential modulation. Thus, the preceding method is unable to estimate the ICFO in one symbol directly and rapidly.
Another technique which detects the fixed position of pilots with boost power in symbols has been proposed. However, such technique does not work well under a condition of a low signal-to-noise ratio (SNR). Furthermore, inaccurate time estimation due to the phase rotation in a frequency domain will also make the frequency offset estimation more difficult.